1. Field of the Invention
The present invention relates to an electronic musical instrument in which the operation cycle for obtaining a musical waveform through utilization of Fourier function synthesizing techniques is changed in accordance with the number of keys being depressed simultaneously, thereby eliminating the discontinuity in temporal variation of the resulting musical tone.
2. Description of the Prior Art
Heretofore there has been a demand for an electronic musical instrument which has a number of tone production lines, each including many tone production channels, to lend realism to each tone. To meet this demand, it is necessary to employ tone production channels, in each of which a musical waveform varies smoothly with time, creating a tone rich in realism. In such tone production channels, however, operations must be performed on a time-shared basis at all times, for which time slots for operation must be secured regardless of actual key depression and release. This will introduce discontinuity in the temporal variation of the musical waveform, causing a reduction in realism of the resulting musical tone and leading to the generation of noise.
FIGS. 4A and 4B are a diagram explanatory of a prior art example, in which a musical tone generating system 100 creates a desired musical tone by use of ordinary Fourier synthesis techniques.
A key tablet assignor 102 scans a key tablet switch group 101 to detect the ON/OFF state, touch response, or similar information of key switches included in the group 101 and holds the information of the respective switches. The information is provided to a control circuit 103 which controls the system 100.
When supplied with the information from the key tablet assignor 102, the control circuit 103 sets a composite waveform in a main memory 108 on the basis of the following Fourier synthesis equation (1): ##EQU1## In the above, q is the harmonic order, n the sample point number, W the number of harmonics, Cq a temporally varying harmonic coefficient, and Zn a sample value. The procedure for the above operation is as follows: A harmonic coefficient generator 105 responds to a control signal from the control circuit 103 to produce the harmonic coefficient Cq of a desired timbre. The harmonic coefficient Cq thus obtained and a q-order sine wave value, ##EQU2## which is read out of a sine wave function table 104 by a signal from the control circuit 103, are multiplied in a multiplier 106. The multiplied value from the multiplier 106 is accumulated by an accumulator 107, by which the composite waveform expressed by Eq. (1) is created and stored in the main memory 108.
Next, the composite waveform thus stored in the main memory 108 is transferred therefrom via a transfer select circuit 109 to at least one of note memories 110-l to 110-m (which can be formed by a single memory through use of time-sharing techniques) corresponding to keys in lines A, B and C. The waveform data thus stored is read out of the corresponding note memories in the respective lines, without exerting any influence upon the composite waveform, by note frequency data from a note frequency data generator 111 which generates note frequency data corresponding to a key being depressed. The waveform data read out from each of the note memories 110-l to 110-m, corresponding to a note scale, is multiplied in one of multipliers 112-l to 112-m, by an envelope output waveform from an envelope generator 113 which generates the envelope waveform corresponding to the depressed key, thus producing musical waveform data added with an envelope. The musical waveform data from the multipliers 112-l to 112 m is converted by D/A converters 114-l to 114-m into an analog waveform, which is applied to a sound system 115, creating a desired musical tone.
The reason for which the lines A, B and C are shown is to indicate that a number of lines are needed in an electronic musical instrument. That is, a number of tone production channels are required in the case of creating different musical tones of several kinds of lines in response to the depression of a single key or in the case of automatic playing of the electronic musical instrument. Now, let it be assumed that the lines A and B each create a musical tone which varies with time (i.e. a musical tone whose harmonic component varies with time) and that the line C creates a musical tone which does not vary with time and whose waveform varies only when a key is newly depressed or when timbre needs to be changed. In such a system it is necessary to always perform waveform operations for the respective note memories of the lines A and B in a repeating cyclic order and write therein the operation results. In this instance, however, when the numbers of note memories in the lines A and B are large, one cycle of operations for all the note memories of the respective lines will take much time. In other words, operations for the note memories of the lines A and B are each performed at long time intervals, with the result that the musical waveform does not smoothely vary with time. When each line includes m note memories as mentioned above, the overall time for operating one musical waveform is 2mT, where T is the time for the waveform operation per note memory.
Where a large number of keys are depressed at the same time, human hearing cannot differentiate individual tones since they are masked by one another, but in the case where a small number of keys are depressed concurrently, individual tones can easily be differentiated from one another. Accordingly, when the number of keys depressed simultaneously during waveform operations at regular intervals is reduced abruptly, there will be created a feeling of discontinuity in the temporal variation of the resulting musical waveform. To avoid this, the present invention changes the operation cycle with the number of depressed keys so that the waveform operation for the depressed keys is quickly performed and immediately followed by the next waveform operation. It is expected that the smoothness of the waveform variation will be improved by switching the above-mentioned operation cycle 2mt to 1/2, .theta., 1/8, . . . in accordance with the number of keys being depressed.